The use of security elements for protecting security documents has been known for some time in a great variety of embodiments. The term “security document” refers here to any document that is to be protected against unauthorized attacks in some way. Documents may have to be protected against different attacks depending on the type of document or information carried by the document.
Certain documents must be protected primarily against forgery and/or falsification. Such documents include for example bank notes, shares, papers of value, ID cards, certificates, checks or virtually any other type of official document. With some of these documents, for example bank notes and papers of value, protection against forgery is most important. Forgery refers here to any imitation of the original, no matter by what means this is attained. In contrast, checks and certificates, for example, must be protected primarily against falsification of content, since such documents are normally not forged completely but primarily subject to the danger of content being altered in fraudulent fashion by e.g. forgery of signatures and replacement of names or amounts.
Typical security elements that are incorporated into the security paper on which the particular document is printed or into the print itself for avoiding forgery are for example watermarks, security threads, mottling fibers, holograms or other special printing processes that are difficult to imitate. Security documents, such as bank notes, are normally equipped with a plurality of different security elements that can be tested partly with the naked eye and partly in special test devices by viewing the documents in reflected light, transmitted light or ultraviolet light.
Despite the great number of existing different security elements, there is a continuing need for new, easily checked, safe means of proof that permit the authenticity of a document or the correctness of its entries to be tested.
A second essential security point is protection against unauthorized perusal and/or duplication of information described on the document by unauthorized persons. Corresponding measures are necessary for example in confidential documents of research and development departments, military documents, minutes of discussions to be kept secret, e.g. at diplomatic services, etc. In particular, since the wide spread of commonly available copying machines already found in most offices and the advances in the area of photography, there is an interest in protecting relevant documents against unauthorized photocopying or photography.
There are various known security elements in this area as well. In the first years following development of the photocopier, confidential documents were covered for example with a red, transparent foil or coating that absorbed the light of the black-and-white copying machines, which was usually green, and thus made a copy impossible. Besides the annoyance due to the difficult readability of documents thus protected, there is the disadvantage that documents are only protected against green light. Photographic images or copying with more modern copying machines with white light are nevertheless possible. In other methods, an embossed transparent foil is applied to the document that acts like a set of prisms and diverts the direction of the light. The prisms can be formed so that one cannot look at the document through them perpendicular from above. This makes it very difficult to make a copy. In newer known methods, phototropic substances are used to protect documents. These are chemical compounds whose structures change under the influence of light. They thus assume a different color and can therefore protect documents against copying in principle. However, it is problematic that such phototropic substances often react quite slowly so that the copy is made faster than the changeover of the substances. Another problem is that such substances cannot prevent documents being nevertheless copied with reduced light or by other methods. A further possibility for preventing unauthorized copying is to install devices in the copying machines that recognize security threads, magnetic features or other protective devices in the paper of the original and then block the copying operation. A great disadvantage of such methods, however, is that they require installations in the machines and these installations can be easily bypassed by more or less informed technicians.
A considerable need for novel security elements therefore also still exists in the area of protection against unauthorized reading or copying of security documents.